Perhaps one of the rarest and most desirable G1 combiners right along side D355: Liokaiser, is Raiden (ライデン; 雷電) - the combined form of 6 Trainbots from the Headmasters series in Japan.
One major reason for its rarity is because it was never released in the US or Europe in any form. The Trainbots were released in a very limited form in Singapore back in 1987 through Japanese retail chains like Isetan and Daimaru (and by that token, probably released in some parts of Asia as well).
As I recall, there were actually a lot less Raidens than Liokaisers back in the day. I only came across Raiden once in Isetan and there was only a set or two. But for months and months, I've seen stacks and stacks of D355: Liokaiser giftsets. I was hardly interested in Liokaiser at the time. He was pretty boring compared to Predaking. So when it came time for mum to buy us something, my brother and I opted for all the Japanese boxed Predacons instead of the Liokaiser giftset (which I must say was actually slightly cheaper than the Japanese boxed Predacons).
I recently obtained a C131: Raiden giftset from my trip in Japan. The story of the acquisition is here. I inspected and studied the Trainbots a fair bit when I returned to Sydney. The Trainbots were all Japanese trains that were unique to Japan at the time (ie: during the "diaclone", pre-Transformers, period). Later, Takara released them as part of the Japanese Headmasters series in 1987.
I think, that is the main issue is this - they were Japanese trains, not futuristic trains or Cybertronian trains, but Japanese trains. Marketing people from various countries would perhaps have suspected that kids outside of Japan may have difficulty identifying with the Trainbots on the same level as they will identify with common road vehicles and or futuristic ones. In all likelihood, Liokaiser which is made up of 3 planes, 2 tanks and a buggy would have been considered more interesting and identifiable to kids and therefore more marketable than 6 trains which look more or less similar (I mean, a train is a train, its a rectagular shaped object without much variation).
Maybe that is why Raiden was not imported, at least in large quantities, into the other Asian countries that had access to "Japanese exclusive" Transformers. Hence its rarity.
Another reason that Raiden is so sought after and complete sets so expensive is because of the number of accessories that make up Raiden. Each train not only came with its own little gun and green connector clip, but also another couple of accessories that are necessary for combination into Raiden. Aside from D318: King Poseidon, where each Seacon came with a number of mostly generic accessories, I think Raiden is the combiner with the most number of accessories in G1.
Here are many photos of my Raiden giftset, definitely one of the jewels of my G1 MIB collection.
C-131: Raiden (ライデン; 雷電)
Function: Heavy Series-Linked Gestalt Warrior
Box: C8.5 (close to C9 is not be slightly damaged flap)
Figures: all C9 at least, tight joints, minimal wear
Stickers: C8.5 for Shouki and Kaen; stickers unapplied for Getsuei, Yukikaze, Suiken and Seizan.
Complete paperworks including giftset instructions, bio card, giftset style sticker sheets (4 sheets still unapplied), catalogue and mail-away exclusive catalogue.
C125: Shouki's sticker sheet (used)
C126: Getsuei and C127: Yukikaze's sticker sheets, unused.
C130: Kaen's sticker sheet (used).
C128: Suiken and C129: Seizan's sticker sheets, unused.
Next: Raiden versus Liokaiser!
Next: Raiden versus Liokaiser!